Glass performance metrics can be accurately assessed at the center of glass (COG) point. This method focuses exclusively on evaluating the performance of the glass materials and the characteristics of the gaps between these components. COG performance provides a clear understanding of how glass functions in isolation, without the influence of external factors. This approach is crucial for determining key performance indicators such as U-value, solar heat gain coefficient (SHGC) and visible light transmittance (VLT).
While COG performance is a significant metric, industry standards require that the overall rating of a window encompasses the entire unit. This includes not only the glass itself but also the edges of the glass and the materials used in the framing. By considering the complete assembly, including how the framing interacts with the glass and contributes to thermal performance, durability and energy efficiency, a more accurate representation of the window's overall quality and performance can be achieved.
"The center of glass U-factor, edge of glass U-factor and the frame U-factor are essential metrics that significantly influence the assessment of a window or glazing system's thermal performance," said Bill Davis, Technical Service & Product Manager at Vitro Architectural Glass. "The center U-factor specifically measures the rate at which heat transfers through the center portion of the glazing material, providing insight into its insulating capabilities. The frame U-factor assesses the energy efficiency of the window frame itself, shedding light on how well it contributes to overall thermal performance. In contrast, the edge of glass U-factor measures the rate at which heat transfers through the vision area near the framing material and includes effects of insulating glass unit spacers, glass, and the adjacent frame material. When these three values are analyzed together on an area weighted basis, they offer a comprehensive understanding of a window's ability to insulate a building effectively.”
COG vs. Frame Differential Values
COG focuses solely on the performance of the glass components and the characteristics of the gaps. In contrast, when evaluating a complete window, the effects of the glass edge system, including the frame, are also considered. Therefore, COG is a useful metric for comparing glass products based on their merits, independent of the impact of the edge system.
To offer a whole window U-value rating, window manufacturers obtain COG U-values from glass manufacturers and thermal conductivity information from frame material and spacer suppliers, using National Fenestration Rating Council (NFRC) procedures to calculate heat transfer rates under standard conditions.
The resulting U-value for the entire window is reported, typically ranging from ~0.20 (very low heat transfer) to 1.2 (high heat transfer) at the NFRC's winter nighttime condition of 0°F exterior and 70°F interior. NFRC labels show U-Factor values in Btu/hr·ft²·°F. However, these ratings focus only on manufacturing performance and do not account for installation methods.
Typically, the U-value of a window unit is higher than that of the glass by itself, and its visible light transmittance (VLT) is reduced because the framing blocks some light. However, this does not imply that window units perform worse than glass alone in all aspects. In fact, window framing materials lower the SHGC of the unit as compared to glass alone because it reduces the amount of transparent material through which solar heat can easily pass compared to non-transparent framing materials.
Vitro’s FramingFactor™
While glass manufacturers can easily calculate COG U-values of their own products, glass edge values are more complicated because they include the thermal conductivity of the insulating glass unit (IGU) spacer system.
To simplify this, Vitro has developed the FramingFactor™ tool, which rapidly calculates the Total Assembly Performance (TAP) for dual-paned and triple-paned IGUs, as well as vacuum insulating glass (VIG) units. FramingFactor™ features various glazing and framing options and allows users to compare performance data quickly and effortlessly. Users can also save, download and share their results.
“This information empowers architects and builders to make well-informed decisions regarding energy efficiency and occupant comfort in their architectural designs,” said Davis.
FramingFactor™ is expected to launch on vitroglazings.com in early 2025.
